1. Field of the Invention
This invention relates to optical systems, and more particularly to an optical system suitable for optically reproducing information stored in a recording medium.
2. Description of the Prior Art
As an apparatus for optically reproducing information, there has heretofore been the so-called optical disk.
The optical disk is such that a semiconductor laser, for example, is employed as a light source, a light beam emergent from the laser being projected on an information medium (disk) by an optical system, whereby information recorded on the disk is reproduced or information is recorded on the disk. FIG. 1 is a diagram showing the schematic construction of a prior-art optical disk apparatus, and principally illustrates an optical system. Referring to FIG. 1, a light beam emitted from a semiconductor laser 1 is guided to a first lens 5 through a prism 20. Here, as shown in the figure the prism 20 is composed of three prisms 2, 3 and 4, which are constructed into a unitary optical component so that the light beam having entered the prism assembly from the side of the semiconductor laser 1 may pass through the prisms 2 and 3 and proceed towards the lens 5 and that a light beam having entered the prism assembly conversely from the side of the lens 5 may be reflected at the joint plane A between the prisms 2 and 3 and proceed towards the prism 4. The light beam guided to the first lens 5 is substantially collimated by this lens, and is projected as a minute light spot on a disk 8 through a second lens 6 which is supported by a voice coil 7. This light beam is reflected by the disk 8, and the reflected beam is received by a photodetector 9 after having passed through the second lens 6, the first lens 5 and the prisms 3 and 4. At this time, if information is recorded on the disk 8 (by way of example, information is recorded by pits which are formed in the disk surface in accordance with the information), the reflected beam has its intensity modulated according to the information, so that the information is obtained in the form of output signals of the photodetector 9. The voice coil 7 serves to move the second lens 6 small distances at a high speed, and is comprised for the purpose of a light spot control, i.e., auto-focusing control or tracking control.
In such apparatus, the laser beam emitted from the semiconductor laser employed as the light source has a divergence or exhibits a ratio of eccentricity being about 3 to 1 and therefore shows an anisotropic (elliptic) far-field pattern. Accordingly, when it is focused on the disk as it is, the beam spot does not become an isotropic (circular) distribution pattern on the disk, and the frequency characteristics of the optical reading of the disk worsen.
In the prior art, therefore, in order to bring a beam spot from a semiconductor laser into a circular distribution in case of employing the semiconductor laser as a light source, the size of the aperture of a lens is appropriately set when a laser beam is transmitted through the lens. That is, by utilizing the numerical aperture (the so-called NA) of the lens, the elliptic distribution pattern is converted into the circular distribution pattern.
With such prior art, however, there is the disadvantage that a lens having a predetermined NA must be used. Further, in case of a low NA, combined lenses (the number of lenses is 2 to 3) must be used, which results in the disadvantage of a complicated construction.
Moreover, in the apparatus of the construction as shown in FIG. 1, outputs from the photodetector must be made sufficiently great in order to detect the information signals at a good signal-to-noise ratio. To this end, the facets of the prism and the lens, for example, one facet 3a of the prism 3 and one facet 5a of the first lens 5 need to be provided with thin films, thereby to prevent the reflection of the light beam and to lessen to the utmost the loss of the light beam due to the prism as well as the lens. In the prior-art optical disk apparatus as shown in FIG. 1, however, the respective constituent elements of the optical system, i.e., the prism and the lenses are all separate, so that the optical efficiency is low and the number of times of the evaporations of the anti-reflection films on the prism, the lens etc. becomes large, resulting in the disadvantage of high cost. Further disadvantages are that since the prism and the lenses are separate, the adjustment of the optical axes of the prism and the lenses is difficult, and that the whole apparatus becomes large on account of the large number of components involved.